1. Field of the Disclosure
The present disclosure relates generally to catalyst materials and processes for making and using them. More particularly, the present disclosure relates to nickel/aluminum-containing catalyst materials useful, for example, as reforming catalysts, processes for making them, and processes for using them in molten carbonate fuel cells.
2. Technical Background
Molten-carbonate fuel cells are high-temperature fuel cells that can produce electrical energy through the net conversion of hydrogen and oxygen to water. The half-reactions are:anode: H2+CO32−H2O+CO2+2e−cathode: ½O2+CO2+2e−CO3 The electrolyte is typically alkali (e.g., Na and K) carbonate retained in a matrix (e.g., a ceramic matrix of LiAlO2. The anode is typically nickel, and the cathode is typically nickel oxide. The CO2 generated at the anode is typically recycled to the cathode where it is consumed.
Such fuel cells typically operate at temperatures on the order of 600-700° C., at which temperatures, the carbonate is a highly conductive molten salt form. Operation at such high temperatures can be advantageous in that it can provide high overall efficiency, even up to 50-60% conversion of the fuel's lower heating value to electricity without recovery and conversion of the exhaust heat. Moreover, the exhaust heat from the fuel cell is relatively hot, and thus may be recovered for the generation of steam, further increasing fuel cell efficiency. Efficiencies in excess of 60% can potentially be achieved with the incorporation of a bottoming cycle.
The hydrogen used in the fuel cell can be provided by a variety of methods. However, in many practical applications, the hydrogen is provided by the reformation of a carbonaceous fuel (e.g., natural gas, methane, petroleum gas, naptha, heavy oil, crude oil) to form hydrogen and CO2. The water-gas shift reaction can be used to provide additional hydrogen. Example reactions for use of methane are provided below:Reformation: CH4+H2O CO+3H2 Water-Gas Shift: CO+H2O CO2+H2 The reforming reaction is typically performed using a nickel catalyst. While part of the reforming is often carried out in a prereformer, in many applications, at least some of the reforming takes place within the molten-carbonate fuel cell itself. This process is known as “direct internal reforming” (DIR). As the reforming reaction is endothermic, it is advantageously performed using the heat generated in the electrochemical reaction. Moreover, the consumption of hydrogen in the cell helps to shift the equilibrium of the reformation reaction to the desired hydrogen product. However, the high operating temperature places severe demands on the corrosion stability and life of cell components. Critically, alkali hydroxides can vaporize from the electrolyte at the high operating temperatures and poison the reforming catalyst.
There remains a need for improved reforming catalysts that are suitable for use in molten carbonate fuel cells and are more resistant to alkali poisoning than conventional catalysts.